This invention relates to a LAN-WAN-LAN communication method whereby communication between LANs is achieved via a wide area network, as well as to a LAN-WAN connecting apparatus. More particularly, the invention relates to a LANN-WAN-LAN communication method, and a LAN-WAN connecting apparatus, in which LAN-WAN-LAN communication is performed by band allocation and line selection in conformity with traffic and type of application.
In order to interconnect LANs (local area networks) that are remote from each other, a so-called wide area network (WAN) such as a public switched telephone network, public data switched network, ISDN or leased line is used as the intermediary. Routers are provided between each LAN and the WAN. The router is connected to the LAN to control physical layers, a data link layer and a network layer, which are first, second and third layers, and is connected to the WAN to control the interface to the WAN as well as the network thereof. When a terminal sends out a frame with an attached network address in a communication system thus interconnecting LANs via a WAN, the router accepts the frame, refers to the network address and sends this frame to the LAN of the other party via the WAN. The router connected to the LAN of the other party accepts the frame and sends it to the LAN, where the destination terminal accepts the frame.
In presently existing WANs, lines have a narrow band. When there is a large increase in traffic in a connection between LANs, the data transfer rate in a WAN suddenly drops to 2400 bps, and this declines to 64 Kbps even when a comparatively fast ISDN Bch is employed. Consequently, a file transfer that ends in several seconds within one and the same LAN is slowed down at the moment the data is passed through the WAN.
Accordingly, consideration has been given to interconnecting LANs by borrowing a fast leased line. However, employing a leased line results in higher communication cost. In particular, since the characteristic of communication using a LAN is such that data does not flow at all times but only when the necessary data is generated (so-called bursty transmission), the fact that there are long periods of idleness between short periods of transmission becomes a problem when a leased line is used.